A drive circuit of the type defined in the opening paragraph is known from U.S. Pat. No. 4,652,984. This known drive circuit is used in a flyback converter, the connection from the primary of the transformer to the first and the second power supply terminal being interrupted periodically by the switching transistor. The starting resistor causes the switching transistor to be turned on, after which the switching transistor is rapidly driven into saturation by positive feedback via the secondary winding, the coupling capacitor and the series resistance. The switching transistor is turned off as soon as the current through this transistor exceeds a given value. The voltage across the secondary winding commutates, i.e. its polarity changes, and positive feedback via the coupling capacitor and the series resistance helps to turn off the switching transistor. The discharge diode is then conductive and provides a discharging path for the coupling capacitor, which speeds up starting of the switching transistor in the next switching cycle.
The switching transistor and the drive circuit can also be used in a flyback converter comprising two switching transistors in a bridge arrangement. The principle of a bridge arrangement is know from inter alia U.S. Pat. No. 4,768,141, FIG. 1. The bridge arrangement comprises two switching transistors, of which the first one can connect one terminal of the primary to the first power supply terminal and of which the second switching transistor can connect the other terminal of the primary to the second power supply terminal. In order to minimize switching losses the two switching transistors must be driven with maximal synchronism. One switching transistor can then be driven in conventional manner, for example with pulse-width control or in a manner as known from the first-mentioned U.S. Pat. No. 4,652,984 and then forms the master switching transistor. The other switching transistor can then be driven by the afore-mentioned known drive circuit and then forms the slave switching transistor because it is turned on and off by the commutation of the voltage on the secondary winding as a result of the master switching transistor being turned on and off.
A disadvantage is now that the slave switching transistor is turned off comparatively slowly because the discharge diode limits the commutated voltage on the control electrode of this switching transistor to approximately 0.7 V. Moreover, the current required for turning off is limited by that part of series resistance situated between the discharge diode and the control electrode of the slave switching transistor. It is an object of the invention to provide a solution enabling the slave switching transistor to be turned off more rapidly.